This invention relates to angular rate-sensing devices, and specifically to an improved tuning fork design useful in such devices.
It is known to provide angular rate-sensing devices incorporating a double tuning fork configuration, one fork having a pair of driven tines and the other fork having a pair of tines for sensing angular rate.
Devices of this type can be provided in either an "open-loop" or a "closed-loop" arrangement. The present invention is useful in either arrangement, and improves the performance of such devices by modifying the frequency of the "pickoff drive mode".
Examples of prior art open-loop devices are illustrated and described in broad terms in U.S. Pat. Nos. 4,524,619 (U.S. Pat. No. Re. 32,931) and 4,899,587 to Staudte and U.S. Pat. No. 4,898,032 to Voles (at FIG. 1a therein and the associated specification). For ease of reference, a copy of Voles FIG. 1a is included herein as FIG. 1a.
An example of a closed-loop prior art design is illustrated in FIG. 1b, which is copied from FIG. 1 of U.S. Pat. No. 5,056,366 to Fersht et al.
Prior art tuning fork devices such as those shown in FIGS. 1a and 1b include many basic features that are preferably also present in the instant invention.
For example, in FIG. 1a, a wafer 11 of piezoelectric material forms a mounting frame for the tuning fork structure. The tuning forks formed by pairs of tines 14, 15 and 16, 17 are interconnected by a stem or base 18, which is in turn connected to the frame 11 by bridges 12 and 13. The "driven" tines 16 and 17 are energized by electrodes operatively attached thereto, and are thereby caused to vibrate in the "x" direction of FIG. 1a, at their natural resonant frequency ("NRF"). Electrodes are also mounted on the "sensing" or "pickoff" tines 14 and 15 to derive the output signal.
When, for example, the structure of FIG. 1a is rotated about the y-axis (while driven tines 16 and 17 are being vibrated in the x-axis direction), Coriolis forces cause torque to pass through the stem 18, causing tines 14 and 15 to vibrate in the direction of the z-axis, which vibration then forms the basis of the output signal.
Additionally similar to the present invention, the structure of FIG. 1a may be chemically etched, machined by laser beam or ultrasonic methods, or similar expedients well known in the semiconductor art.
For several reasons (known to persons of ordinary skill in the art), it is desirable and advantageous to tune the sensing fork NRF to near the NRF of the driven fork. see, for example, U.S. Pat. No. 5,056,366 to Fersht et al., at col. 2, 1. 2-12. In an open-loop system, the difference in these frequencies is greater than the bandwidth. Typical design requirements for such double tuning fork configurations "open-loop systems" place the "drive" and the "pickoff" mode NRFs near each other at about 10 KHz.
In closed-loop systems, the difference in these frequencies can be less than the bandwidth, and can even be zero. Those skilled in the art will understand that, in closed-loop systems such as Fersht, additional control electrodes are used to "null" the Coriolis-induced out-of-plane vibration of the tines. Through a closed-loop feedback system, the Coriolis forces experienced by the driven tines is measured and utilized to determine the rate of rotation about the y-axis of the sensor.
Thus, in both open- and closed-loop systems the NRFs of the drive mode and the pickoff mode are near to each other.
In devices of this type, it is also desirable to locate the double tuning fork free-body motion nodes of the pickoff mode and the drive mode at the points at which the tuning fork structure is attached to the frame. In FIG. 1a, for example, these attachment points are bridges 12 and 13, although those skilled in the art will understand that the device of FIG. 1a does not locate the relevant nodes at those attachment points because of the lack of symmetry of the fork structure.
By locating these nodes at those attachment points, the forks can be better isolated from external stresses, twists, and similar forces on the frame 11 that might otherwise interfere with the desired measurements of the tuning fork device.
In other words, the risk of errors from such external stresses and vibrations can be minimized by placing the node of pickoff mode motion along the longitudinal axis of a connecting bridge, such as bridges 12 and 13 (FIG. 1a). One way to accomplish this node placement is to make the tuning forks "mirror images" of each other.
The need for the present invention arises, however, because of additional design requirements for these devices. Among other things, in order for the output signals to be readily ascertainable, all other normal mode NRFs (other than the drive and pickoff modes) must typically be separated from the normal drive NRF by at least 2 KHz. One such "other" mode which can be especially troublesome in this regard is the normal "pickoff drive mode" (the mode in which the pickoff tines vibrate directly toward and away from each other).
In the aforementioned "mirror-image" configuration, the "sensing" fork has a "pickoff drive mode" with a NRF that is identical or very near the "actual" drive mode NRF. Thus, although the "mirror-image" configuration achieves the desired isolation of the tuning forks from external forces, it also violates the aforementioned 2 KHz separation rule and renders the device unsuitable for its intended sensing purpose. On the other hand, the desired 2 KHz separation can be achieved by altering the configuration and/or dimensions of one of the forks (so that they are not "mirror-images" of each other; see FIG. 1c, for example), but then the connecting bridge structure is not the motion node (that is, the motion node is offset from the axis of the connecting bridge), and external stresses and undesired vibrations from the mounting wafer frame 11 can alter the pickoff mode, causing errors in the output signal.
Those skilled in the art will understand that, in certain prior art designs, complicated structures are incorporated into the frame 11 (FIG. 1a) to try to isolate the sensor from such external stresses and undesired vibrations. Prior art designs, however, have been unable to simultaneously accomplish both of the foregoing objectives: to place the node of pickoff mode motion along the longitudinal axis of the connecting bridge, such as bridges 12 and 13 (FIG. 1a); and to meet the foregoing 2 KHz separation criteria for the "pickoff drive mode".